An optical disk is used as a mass storage device and an optical disk unit is used for recording or reproducing information to or from the optical disk. Known types of optical disks include a read-only compact disk (CD) for reproduction, a write-once optical disk (WO) for writing information to the optical disk just once and a magneto optical disk (MO) in which information can be freely recorded or reproduced. In these optical disks, each track is arranged in a fine spiral form in a direction from an inner circumference to an outer circumference forming a direction of recording or reproducing information.
The optical disk unit for recording or reproducing information to or from the optical disk generally includes a turntable directly connected to a driving motor for driving rotation of the optical disk. An optical head for recording or reproducing information to or from the optical disk has a head base, on which a light emitting element, a light receiving element, and an object lens are mounted. The object lens is supported by a lens driving mechanism so as to have a free displacement in a radial and axial direction of the optical disk, and the entire optical head is supported by a head carrying sledge mechanism so as to move freely in a radial direction of the optical disk.
There are two operational steps in recording or reproducing information to or from the optical disk by means of the optical disk unit; one is a target retrieval operation in which an optical head moves to a target position for a recording or reproducing operation and the other is the recording/reproducing operation itself. In other words, the optical head is first moved to a target position of the optical disk by means of the head carrying sledge mechanism in the target retrieval operation, and optical scanning is then executed on the track of the rotationally driven optical disk by means of the optical head in the recording/reproducing operation.
During optical scanning, a minute spot is focused on the track of the optical disk by concentrating emitted light of the light emitting element by means of the object lens, the object lens is appropriately displaced radially by means of the lens driving mechanism in accordance with a minute displacement of the track, and when the limit of the displacement of the object lens is near, the entire optical head is moved by means of the head carrying sledge mechanism and the object lens returned by the lens driving mechanism to a position where it again begins to be displaced as it follows the track. Information is recorded or reproduced to or from the optical disk driving the scanning operation.
If information is recorded into the optical disk by this type of optical disk unit, however, its beginning part may sometimes be lost. This is because when information is recorded into the optical disk, the head carrying mechanism moves the optical head to a target position of the optical disk, and then moves the optical head in the forward direction to record information. If these movement directions are opposite to each other, the object lens is shocked or vibrated when the optical head quickly turns around.
A lens driving mechanism supports the object lens on a lightweight movable part so as to have a free displacement with a low resistance since it is required to displace the object lens precisely and at a high speed and therefore it is not structurally resistant to an external disturbance such as vibration or a shock. On the other hand, a sledge driving mechanism is formed in a simple structure by means of a commercially available servo motor and a gear. Since it serves as a mechanism for moving roughly the entire optical head, it causes a great vibration or shock at the beginning of a carrying operation or when turned around.
Accordingly, if a track of the optical disk is optically scanned from a target position by means of the optical head, the optical scanning may not be favorably started if the moving direction of the optical head is inverted when the scanning operation is started. In this case, the beginning of the information to be recorded may be lost, and in the same manner, an error may also occur during reproduction of the information.